1. Field of the Invention
The present invention relates in general to a method to fabricate a flat panel display with thin film transistors. In particular, the present invention relates to a liquid crystal display (hereinafter LCD) formed using the light from the back of a substrate for exposure to simplify the fabricating process.
2. Description of the Related Art
The conventional active matrix LCD comprises a plurality of pixel electrodes and switching devices, for example, thin film transistors (hereinafter TFT). The pixels are defined by connected gate lines and data lines. Each pixel comprises a storage capacitor and a pixel electrode connected to the switching devices.
The conventional fabricating process of the TFT of LCD comprises the following steps. First, in FIG. 1A, a first metal film is formed on a transparent substrate 40. Next, a first mask pattern is used to form the gate metal line 42 on the first metal film. Next, in FIG. 1B, an insulating layer 44a is formed on the substrate 40. Next, an amorphous silicon layer 44b is formed on the insulating layer 44a. Next, an n-type doped layer 44c is formed on the amorphous silicon layer 44b. Next, a conductor layer 46 is formed on the n-type doped layer 44c. Next, a photoresist layer 48 is formed on the conductor layer 46. Next, in FIG. 1C, patterns are defined by performing exposure with a second mask and a photo etching process. This step forms an island structure 52 composed of the amorphous silicon layer 44b, the n-type doped layer 44c, and the conductor layer 46 using the photoresist layer 48a on the gate metal line 42. Next, in FIG. 1D, the n-type doped layer 44c and the conductor layer 46 on the gate metal line 42 are removed by performing exposure with a third mask and photo etching process to form a channel 53, the drain electrode 54 and a source electrode 56. Next, a protecting thin-film and a second photoresist layer are formed. In FIG. 1E, a protecting layer 58 is formed by performing exposure with a fourth mask and photo etching process. The protecting layer 58 covers the drain electrode 54, the source electrode 56, the amorphous silicon layer 44b, and the insulating layer 44a. The protecting layer 58 further comprises a plurality of openings (58a, 58b). The drain opening 58a is on the drain electrode 54, and the source opening 58b is on the source electrode 56. Finally, in FIG. 1F, the pixel electrode comprises a drain pixel electrode 62 and a source pixel electrode 64. The drain pixel electrode 62 contacts the drain electrode 54 through the drain opening 58a and the source pixel electrode 64 contacts the source electrode 56 through the source opening 58b. 
However, the conventional process requires five performances of photo etching process. The cost and efficiency can be greatly improved if the steps of the photo etching process can be reduced to four.
To achieve the above-mentioned object, the method to fabricate thin film transistors of a flat panel display comprises the following steps. First, a gate metal line is formed on the substrate. Next, an insulating layer, a semiconductor layer, a doped silicon layer, and a signal metal line are formed on the substrate and the gate metal line. Next, the doped silicon layer and the signal metal line are defined to form a drain electrode and a source electrode, then a channel between the drain electrode and the source electrode is defined. Next, a photoresist layer is formed on the semiconductor layer, the drain electrode, the source electrode, and the channel. Next, the photoresist layer is exposed by the light from the bottom of the substrate, and then the exposed photoresist layer is removed. Then, the semiconductor layer is removed using the remaining photoresist layer as a mask to form an island structure containing the semiconductor layer, the doped silicon layer, the drain electrode, and the source electrode. Next, the photoresist layer is removed and a protecting layer is formed on the insulating layer, the drain electrode, the source electrode, and the semiconductor layer. The protecting layercomprises an opening on the drain electrode or source electrode. Finally, a pixel electrodeis formed on the protecting layer, wherein the pixel electrode contacts the drain electrode through the opening.
Further, the method to fabricate thin film transistors of a flat panel display comprises the following steps. First, a gate metal line is formed on the transistor region of the substrate. Next, an insulating layer, a semiconductor layer, a doped silicon layer, and a signal metal line are formed on the gate metal line. Next, the doped silicon layer and the signal metal line are defined to form a drain electrode and a source electrode, and a channel is defined between the drain electrode and the source electrode. Next, a photoresist layer is formed on the semiconductor layer, the drain electrode, the source electrode, and the channel. Next, the photoresist layer is exposed by the light from the bottom of the substrate, then the exposed photoresist layer is removed. Next, the semiconductor layer is removed using the remaining photoresist layer as a mask to form an island structure containing the semiconductor layer, the doped silicon layer, the drain electrode, and the source electrode. Next, the photoresist layer is removed, and a pixel electrode is formed on the insulating layer and the drain electrode, wherein the pixel electrode covers the drain electrode. Finally, a protecting layer is formed on the drain pixel electrode and the insulating layer.
The characteristic of the present invention is that the method to fabricate the TFT of a flat panel display includes four photo etching processes and a single back-exposure step. The gate is formed by the first photo etching process. The source and the drain are formed by the second photo etching process. Next, the exposure is performed from the back of the substrate to form an island structure. Then, a protecting layer is formed by the third photo etching process. Finally, the pixel electrode is formed by the fourth photo etching process.